勒拼音-烹鲜成语

Chapter 2 Basic concepts
There are some basic concepts about networks we need to know.

2.1 Line configuration
Line configuration refers to the way two or more communication devices attach to a link. A link is the physical communication pathway (wired or wireless) that transfers data from one device to another. Another term channel refers the logical pathway. Sometime they could refer the same thing- a medium. Generally, we could say that a link contains one or more channels, and a channel contains a data stream. For the purpose of visualization, it is simplest to imagine any link as a line drawn between two points. For communication to occur, two devices must be connected in some way to the same link at the same time. There are two possible line configurations: point-to-point and multipoint.
Point-to-point
A point-to-point line configuration provides a dedicated link between two devices. The entire capacity of the channel is reserved for transmission between those two devices. Most point-to-point line use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible, as shown in Figure 2.1. When you change television channels by infrared remote control, you are establishing a point-to-point line configuration between the remote control and the television’s control system.

Figure 2.1 Point-to-point line configurations
Multi-point
A multipoint (also called multi-drop) line configuration is one in which more than two specific devices share a single link, as shown in Figure 2.2

Figure 2.2 Multipoint
In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared line configuration. If users must take turns, it is a time shared line configuration.
2.2 Topology
The term topology refers to the way a network is laid out, either physically or logically. Two or more devices connect to a link (called line configuration); two or more links form a topology. The topology of a network is the geometric representation of the relationship all the links and linking devices (usually called nodes) to each other. There are five topologies possible: mesh, star, tree, bus, and ring.
These five labels describe how the devices in a network are interconnected rather than their physical arrangement. For example, having a star topology does not mean that all of the computers in the network must be placed physically around a hub in a star shape. A consideration when choosing a topology is the relative status of the devices to be linked. Two relationships are possible: peer-to-peer, where the devices share the link equally, and primary-secondary, where one device controls traffic and the others must transmit through it. Ring and mesh topology are more convenient for peer-to-peer transmission, while star and tree more convenient for primary-secondary. A bust topology is equal
ly convenient for either. But this is not the only consideration. There are many factors contribute the selection of topology.
Mesh
In a mesh topology, every device has a dedicated point-to-point link to every other device (also called intact mesh, if there is not a link every between two devices, it called part-intact mesh topology). The term dedicated means that the link carries traffic only between the two devices it connects. A fully connected mesh network has n(n-1)/2 physical channels to link n devices. To accommodate that many links, every device on the network must have n-1 input/output (I/O) ports, as shown in Figure 2.3.

Figure 2.3 A fully connected mesh topology for five devices
A mesh offers several advantages over other network topologies. First, the use of dedicated links guarantees that each connection can carry its data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices. Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system. And the third is privacy or security. When every message sent gravels along a dedicated line, only the intended recipient can see it. Physical boundaries prevent other users from gaining access to messages. Finally, point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems. This facility enables the network manager to discover the precise location of the fault and aids in finding its cause and solution.
The main disadvantages of a mesh topology are related to the amount of cabling and the number of I/O ports required. First, because every device must be connected to every other device, installation and reconfiguration are difficult. Second, the sheer bulk of the wiring can be greater that the available space (in walls, ceilings, or floor) can accommodate. And, finally, the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. For these reason a mesh topology is usually implemented in a limited fashion, for example, as backbone connecting the main computers of a hybrid network that can include several other topologies
Example 2.1
The library building has a fully connected mesh network consisting of eight devices. Calculate the total number of cable links needed and the number of ports for each device.
Solution
Number of links=n(n-1)/2=8(8-1)/2=28
Number of ports per device=n-1=8-1=7
Star
In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not linked to each other directly. Unlike a mesh topology, a star does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends to the controller, which then relays the data to the other connected devices, as shown in Figure 2.4.

Figure 2.4 Star topology
A star topology is less expen
sive than a mesh topology. In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions, moves, additions involve only one connection-between that device and the hub.
Other advantages include robustness. If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links.
However, although a star requires far less cable than a mesh, each node must be connected to a central hub. For this reason more cabling is required in a star than in some other topologies, such as tree, ring and bus.
Tree
A tree topology is a variation of a star. As in a star, nodes in a tree are linked to a central hub that controls the traffic to the network. However, not every device plugs directly into the hub. The majority of devices connect to a secondary hub that in turn is connected to the central hub, as shown in Figure 2.5.

Figure 2.5 Tree topology
The central hub in the tree is an active hub. An active hub contains a repeater, which is a hardware device that regenerates the received bit patterns before sending them out. Repeating strengthens transmissions and increases the distance a signal can travel between sender and receiver.
The secondary hubs may be active or passive hubs. A passive hub provides a simple physical connection between the attached devices. Internally, each passive hub contains a set of resistors to balance the circuit linking the connected devices.
The advantages and disadvantages of a tree topology ate generally the same as those of a star. The addition of secondary hubs, however, brings two further advantages, First, it allows more devices to be attached to a single central hub and can therefore increase the distance a signal can travel between devices. Second, it allows the network to isolate and prioritize communications from different computers. For example, the computers attached to one secondary hub can be given priority over computers attached to another secondary hub. In this way the network designers and operator can guarantee that the time-sensitive data will not have to wait for access to the network.
Bus
The preceding examples all describe point-to-point configurations. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in the network, as shown in Figure 2.6.

Figure 2.6 Bus topology
Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into
heat. Therefore, it becomes weaker and weaker the farther it has to travel. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.
Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, and then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh, star, or tree topologies. In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub. In a bus, this redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone.
Disadvantages include difficult reconfiguration and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices. As mentioned above, signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable. Adding new devices may therefore require modification or replacement of the backbone.
In addition, a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions.
Ring
In a ring topology, each device has a dedicated point-to-point line configuration only with the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along, as shown in Figure 2.7.

Figure 2.7 Ring topology
A ring is relatively easy to install and reconfigure. Each device is linked only to its immediate neighbors (either spatially or logically). To add or delete a device requires moving only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices). In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location.
However, unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break.
Example 2.2
If the devices in Example 2.1 are configured as a ring instead of a mesh, how many cable links are required?
Solution
To connect n devices in a ring topology, we need n cable links. An eight-device ring needs eight cable links.
Hybrid
urces to be shared can include hardware (e.g., a printer), software (e.g., an application program), or data. A common example of a LAN, found in many business environments, links a work group of task-related computers, for example, engineering workstations or accounting PCs. One of the computers may be given a large-capacity disk drive and become a server to the other clients. Software can be stored on this central server and used as needed by the whole group. In this example, the size of the LAN may be determined by licensing restrictions on the number of users per copy of software, or by restrictions on the number of users licensed to access the operating system.
In addition to size, LANs are distinguished from other types of networks by their transmission media and topology. In general, a given LAN will use only one type of transmission medium. The most common LAN topologies are bus, ring, and star.
Traditionally, LANs have data rates in the 5 to 100 Mbps range. Today, however, speeds are increasing and can reach 1 Gbps, with the 10 Gbps systems in development. LANs are discussed at length in following chapter**.
Metropolitan Area Network (MAN)
A metropolitan area network is designed to extend over an entire city. It may be a single network such as a cable television network, or it may be a means of connecting a number of LANs into a larger network so that resources may be shared LAN-to-LAN as well as device-to-device. For example, a company can use a MAN to connect the LANs in all of its offices throughout a city.
Wide Area Network (WAN)
A wide area network provides long-distance transmission of data, voice, image, and video information over large geographical areas that may comprise a country, a continent, or even the whole world.
In contrast to LANs (which depend on their own hardware for transmission), WANs may utilize public, leased, or private communication devices, usually in combinations, and can therefore span an unlimited number of miles.
2.5 Internet(works)
When two or more networks are connected, they become an inter-net-work, or internet (see Figure 2.12; in the figure, the boxes labeled R represent routers). Individual networks are joined into internetworks by the use of internetworking devices. These devices which include routers and gateways are discussed in the later Chapter**. The term internet (lowercase i) should not be confused with the Internet (uppercase I). The first is a generic term used to mean an interconnection of networks. The second is the name of a specific worldwide network, an instance of internet.

Figure 2.12 internetwork (internet)

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